Just now I wanted to explain why I was doing something, then I wrote "the purpose of doing something is blablabla". Immediately I wondered why I didn't write "the reason of doing something is blablabla" instead. It seems to me that the words purpose and reason are perfectly interchangeable in this context. On the other hand, however, the word purpose impresses me as "where is it going" while the word reason carries the connotation of "where is it coming from". In this sense, they seem to contradict each other.

Any one can explain this phenomenon to me? Maybe it is only me that is confused due to misconception of the two words.

It's just that reason and purpose happen to overlap in the sense of "motivation", making them interchangeable in this particular context. The reason = "where is it coming from" sense is completely separate - it's not a "connotation" that has any bearing on the choice made in this context.
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FumbleFingersNov 23 '12 at 18:10

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@FumbleFingers Perhaps there is a difference in describing something looking backwards to its past versus forwards to its future. What originally/initially caused this or that, versus what will this eventually cause. In some languages, this is clearer than it always is in English.
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tchristNov 23 '12 at 18:58

@tchrist: I agree there's a real-world difference between "this [now] because of that [in the past]", and "the other [in the future] because of this [now]". But it's not obvious to me why this should lead to any confusion about cause and effect, even if we happen to use the same words (reason, because, or just plain as) when linking the two.
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FumbleFingersNov 23 '12 at 21:14

2 Answers
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I was inclined to suggest that this was a general reference question, yet the answer wasn't obvious to me either! Here we go, in a variety of contexts, which is why the question is of interest to me too.

To do something for a purpose means you're doing it to accomplish
something else, which is your PURPOSE. Accomplishing a purpose is only
one of many reasons why we do something--in this case, REASON is more
like "motive."

A reason can be trivial or illogical. One can purchase something for the reason that one likes how it looks e.g. I might buy sky-blue colored suede, open-toe, high-heel pumps because they're cute, but that won't accomplish the ultimate purpose in buying footwear, which is to facilitate walking.

Philosophical
The reason for which something is done, or the reason it is done in a particular way, is the purpose. Reason motivates action.

Grammatical
Purpose and reason are neither synonyms, nor antonyms, which was part of the question.
As a part of speech, both words are nouns. That is the similarity. This is the difference:

Purpose is the reason for which something is done or created or for
which something exists while Reason is a cause, explanation or
justification for an action or event.

Side bar: It is more appropriate to say "The reason for blah is blahblah". Usage is context-specific with "purpose" e.g. "The purpose of blah..." versus "The purpose for blah...".

In my view the Purpose is what you want to achieve (e.g. become a millionaire or become a moviestar), the Reason is why you want to achieve it (e.g. you want to live free of financial worries or you like to entertain people).

Consider this quote:

The purpose of life is to live it, to taste experience to the utmost, to reach out eagerly and without fear for newer and richer experience.” ― Eleanor Roosevelt

Here, the purpose is to really live and the reason why is to have enriching experiences.